A modulating signal to be used for modulating a transmitter may be viewed as having a reference level (the DC part of the signal) and an amplitude varying with time about that reference level. Often this modulating signal, for example analog voice, will have a reference level of "0" or a none zero reference level that is of no consequence to users of the signal. When this reference level is of consequence to users of the signal additional problems may be presented to practitioners that wish to send this signal over, for example, a wireless communications system. In such circumstances not only must the original reference level be recovered but additional problems introduced by the communications system must be eliminated.
For example in a wireless communications system relying on radio waves, the modulating signal is typically imposed on a radio signal as frequency variation about an assigned transmit frequency. For any number of practical reasons the actual frequency of a transmitted radio signal will often vary slightly from the assigned frequency. This variation may be referred to as a transmit error. Similarly a receiver bent on receiving this radio signal, while nominally tuned to the assigned frequency, will usually introduce a receive error (variation from the assigned frequency). These frequency errors in combination may result in a reference level error or offset at the receiver output. When the modulating signal requires the reference level for accurate demodulation, for example a data signal, this reference level error or undesired offset will usually need to be eliminated.
To this end practitioners have estimated a reference level by averaging a maximum amplitude estimate and a minimum amplitude estimate. This approach, without more, works reasonably well unless and until a pattern within the signal is encountered. A pattern in for example a data signal is experienced when the data signal has a desired, nonzero offset over a time period of consequence, e.g. a run of a particular nonzero data character. Under such circumstances either the maximum or minimum amplitude estimate tends to decay causing an error in the estimated reference level. When this error becomes significant, depending in turn on the precise data signal, etc. an unacceptable degree of inaccuracy in the demodulated signal may result.
Another solution at least conceptually, suggests locking the receiver frequency to the transmitter frequency to help eliminate the reference level offsets. Unfortunately this approach is generally considered impractical due to the degree of complexity involved. To date the absence of practical solutions, if not paucity of solutions, has often resulted in practitioners resorting to simply disallowing modulating signals with desired offsets beyond some boundary. Such disallowance amounts to tacit acceptance of an otherwise unnecessary limitation on the communications system.
Clearly a need exists for a method and apparatus that accurately determines an undesired offset in a signal, such method and apparatus being further capable of pattern adaptive offset restoration.